Convert CONFIG_ENV_SPI_* to Kconfig

This converts the following to Kconfig:
  CONFIG_ENV_SPI_BUS
  CONFIG_ENV_SPI_CS
  CONFIG_ENV_SPI_MAX_HZ
  CONFIG_ENV_SPI_MODE

Most of time these value are not needed, CONFIG_SF_DEFAULT
with same value is used, so I introduced CONFIG_USE_ENV_SPI_*
to force the associated value for the environment.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/da850_am18xxevm_defconfig b/configs/da850_am18xxevm_defconfig
index 4192241..b8c16ba 100644
--- a/configs/da850_am18xxevm_defconfig
+++ b/configs/da850_am18xxevm_defconfig
@@ -39,6 +39,10 @@
 CONFIG_DEFAULT_DEVICE_TREE="da850-evm"
 CONFIG_SPL_OF_PLATDATA=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_USE_ENV_SPI_MAX_HZ=y
+CONFIG_ENV_SPI_MAX_HZ=0
+CONFIG_USE_ENV_SPI_MODE=y
+CONFIG_ENV_SPI_MODE=0
 CONFIG_DM=y
 CONFIG_SPL_DM=y
 CONFIG_DA8XX_GPIO=y